Network Organization and Architecture

1
Chapter 11

• Network Organization and Architecture

2
Chapter 11 Objectives

• Become familiar with the fundamentals of network
  architectures.
• Learn the basic components of a local area
  network.
• Become familiar with the general architecture of
  the Internet.

3
11.1 Introduction

• The network is a crucial component of todays
  computing systems.
• Resource sharing across networks has taken the
  form of multitier architectures having numerous
  disparate servers, sometimes far removed from the
  users of the system.
• If you think of a computing system as collection
  of workstations and servers, then surely the
  network is the system bus of this configuration.

4
11.2 Early Business Computer Networks

• The first computer networks consisted of a
  mainframe host that was connected to one or more
  front end processors.
• Front end processors received input over
  dedicated lines from remote communications
  controllers connected to several dumb terminals.
• The protocols employed by this configuration were
  proprietary to each vendors system.
• One of these, IBMs SNA became the model for an
  international communications standard, the
  ISO/OSI Reference Model.

5
11.3 Early Academic and Scientific Networks

• In the 1960s, the Advanced Research Projects
  Agency funded research under the auspices of the
  U.S. Department of Defense.
• Computers at that time were few and costly. In
  1968, the Defense Department funded an
  interconnecting network to make the most of these
  precious resources.
• The network, DARPANet, designed by Bolt, Beranek,
  and Newman, had sufficient redundancy to
  withstand the loss of a good portion of the
  network.
• DARPANet, later turned over to the public domain,
  eventually evolved to become todays Internet.

6
11.4 Network Protocols I ISO/OSI Reference Model

• To address the growing tangle of incompatible
  proprietary network protocols, in 1984 the ISO
  formed a committee to devise a unified protocol
  standard.
• The result of this effort is the ISO Open Systems
  Interconnect Reference Model (ISO/OSI RM).
• The ISOs work is called a reference model
  because virtually no commercial system uses all
  of the features precisely as specified in the
  model.
• The ISO/OSI model does, however, lend itself to
  understanding the concept of a unified
  communications architecture.

7
11.4 Network Protocols I ISO/OSI Reference Model

• The OSI RM contains seven protocol layers,
  starting with physical media interconnections at
  Layer 1, through applications at Layer 7.

8
11.4 Network Protocols I ISO/OSI Reference Model

• OSI model defines only the functions of each of
  the seven layers and the interfaces between them.
  
• Implementation details are not part of the model.
  

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11.4 Network Protocols I ISO/OSI Reference Model

• The Physical layer receives a stream of bits from
  the Data Link layer above it, encodes them and
  places them on the communications medium.
• The Physical layer conveys transmission frames,
  called Physical Protocol Data Units, or Physical
  PDUs. Each physical PDU carries an address and
  has delimiter signal patterns that surround the
  payload, or contents, of the PDU.

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11.4 Network Protocols I ISO/OSI Reference Model

• The Data Link layer negotiates frame sizes and
  the speed at which they are sent with the Data
  Link layer at the other end.
• The timing of frame transmission is called flow
  control.
• Data Link layers at both ends acknowledge packets
  as they are exchanged. The sender retransmits the
  packet if no acknowledgement is received within a
  given time interval.

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11.4 Network Protocols I ISO/OSI Reference Model

• At the originating computers, the Network layer
  adds addressing information to the Transport
  layer PDUs.
• The Network layer establishes the route and
  ensures that the PDU size is compatible with all
  of the equipment between the source and the
  destination.
• Its most important job is in moving PDUs across
  intermediate nodes.

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11.4 Network Protocols I ISO/OSI Reference Model

• the OSI Transport layer provides end-to-end
  acknowledgement and error correction through its
  handshaking with the Transport layer at the other
  end of the conversation.
• The Transport layer is the lowest layer of the
  OSI model at which there is any awareness of the
  network or its protocols.
• Transport layer assures the Session layer that
  there are no network-induced errors in the PDU.

13
11.4 Network Protocols I ISO/OSI Reference Model

• The Session layer arbitrates the dialogue between
  two communicating nodes, opening and closing that
  dialogue as necessary.
• It controls the direction and mode (half -duplex
  or full-duplex).
• It also supplies recovery checkpoints during file
  transfers.
• Checkpoints are issued each time a block of data
  is acknowledged as being received in good
  condition.

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11.4 Network Protocols I ISO/OSI Reference Model

• The Presentation layer provides high-level data
  interpretation services for the Application layer
  above it, such as EBCDIC-to-ASCII translation.
• Presentation layer services are also called into
  play if we use encryption or certain types of
  data compression.

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11.4 Network Protocols I ISO/OSI Reference Model

• The Application layer supplies meaningful
  information and services to users at one end of
  the communication and interfaces with system
  resources (programs and data files) at the other
  end of the communication.
• All that applications need to do is to send
  messages to the Presentation layer, and the lower
  layers take care of the hard part.

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11.4 Network Protocols II TCP/IP Architecture

• TCP/IP is the de facto global data
  communications standard.

• It has a lean 3-layer protocol stack that can be
  mapped to five of the seven in the OSI model.
• TCP/IP can be used with any type of network, even
  different types of networks within a single
  session.

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11.4 Network Protocols II TCP/IP Architecture

• The IP Layer of the TCP/IP protocol stack
  provides essentially the same services as the
  Network and Data Link layers of the OSI Reference
  Model.
• It divides TCP packets into protocol data units
  called datagrams, and then attaches routing
  information.

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11.4 Network Protocols II TCP/IP Architecture

• The concept of the datagram was fundamental to
  the robustness of ARPAnet, and now, the Internet.
  
• Datagrams can take any route available to them
  without human intervention.

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11.4 Network Protocols II TCP/IP Architecture

• The current version of IP, IPv4, was never
  designed to serve millions of network components
  scattered across the globe.
• It limitations include 32-bit addresses, a packet
  length limited to 65,635 bytes, and that all
  security measures are optional.
• Furthermore, network addresses have been assigned
  with little planning which has resulted in slow
  and cumbersome routing hardware and software.
• We will see later how these problems have been
  addressed by IPv6.

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11.4 Network Protocols II TCP/IP Architecture

• Transmission Control Protocol (TCP) is the
  consumer of IP services.
• It engages in a conversation-- a connection--
  with the TCP process running on the remote
  system.
• A TCP connection is analogous to a telephone
  conversation, with its own protocol "etiquette."

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11.4 Network Protocols II TCP/IP Architecture

• As part of initiating a connection, TCP also
  opens a service access point (SAP) in the
  application running above it.
• In TCP, this SAP is a numerical value called a
  port.
• The combination of the port number, the host ID,
  and the protocol designation becomes a socket,
  which is logically equivalent to a file name (or
  handle) to the application running above TCP.
• Port numbers 0 through 1023 are called
  well-known port numbers because they are
  reserved for particular TCP applications.

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11.4 Network Protocols II TCP/IP Architecture

• TCP makes sure that the stream of data it
  provides to the application is complete, in its
  proper sequence and that no data is duplicated.
• TCP also makes sure that its segments arent sent
  so fast that they overwhelm intermediate nodes or
  the receiver.
• A TCP segment requires at least 20 bytes for its
  header. The data payload is optional.
• A segment can be at most 65,656 bytes long,
  including the header, so that the entire segment
  fits into an IP payload.

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11.4 Network Protocols II TCP/IP Architecture

• In 1994, the Internet Engineering Task Force
  began work on what is now IP Version 6.
• The IETF's primary motivation in designing a
  successor to IPv4 was, of course, to extend IP's
  address space beyond its current 32-bit limit to
  128 bits for both the source and destination host
  addresses.
• This is a seemingly inexhaustible address space,
  giving 2128 possible host addresses.
• The IETF also devised the Aggregatable Global
  Unicast Address Format to manage this huge
  address space.

24
11.4 Network Protocols II TCP/IP Architecture

• In 1994, the Internet Engineering Task Force
  began work on what is now IP Version 6.
• The IETF's primary motivation in designing a
  successor to IPv4 was, of course, to extend IP's
  address space beyond its current 32-bit limit to
  128 bits for both the source and destination host
  addresses.
• This is a seemingly inexhaustible address space,
  giving 2128 possible host addresses.
• The IETF also devised the Aggregatable Global
  Unicast Address Format to manage this huge
  address space.

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11.6 Network Organization

• Computer networks are often classified according
  to their geographic service areas.
• The smallest networks are local area networks
  (LANs). LANs are typically used in a single
  building, or a group of buildings that are near
  each other.
• Metropolitan area networks (MANs) are networks
  that cover a city and its environs.
• LANs are becoming faster and more easily
  integrated with WAN technology, it is conceivable
  that someday the concept of a MAN may disappear
  entirely.
• Wide area networks (WANs) can cover multiple
  cities, or span the entire world.

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11.6 Network Organization

• In this section, we examine the physical network
  components common to LANs, MANs and WANs.
• We start at the lowest level of network
  organization, the physical medium level, Layer 1.
  
• There are two general types of communications
  media Guided transmission media and unguided
  transmission media.
• Unguided media broadcast data over the airwaves
  using infrared, microwave, satellite, or
  broadcast radio carrier signals.

27
11.6 Network Organization

• Guided media are physical connectors such as
  copper wire or fiber optic cable that directly
  connect to each network node.
• The electrical phenomena that work against the
  accurate transmission of signals are called
  noise.
• Signal and noise strengths are both measured in
  decibels (dB).
• Cables are rated according to how well they
  convey signals at different frequencies in the
  presence of noise.

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11.6 Network Organization

• The signal-to-noise rating, measured in decibels,
  quantifies the quality of the communications
  channel.
• The bandwidth of a medium is technically the
  range of frequencies that it can carry, measured
  in Hertz.
• In digital communications, bandwidth is the
  general term for the information-carrying
  capacity of a medium, measured in bits per second
  (bps).
• Another important measure is bit error rate
  (BER), which is the ratio of the number of bits
  received in error to the total number of bits
  received.

29
11.6 Network Organization

• Coaxial cable was once the medium of choice for
  data communications.
• It can carry signals up to trillions of cycles
  per second with low attenuation.
• Today, it is used mostly for broadcast and closed
  circuit television applications.

_
Coaxial cable also carries signals for
residential Internet services that piggyback on
cable television lines.
30
11.6 Network Organization

• Twisted pair cabling, containing two twisted wire
  pairs, is found in most local area network
  installations today.
• It comes in two varieties shielded and
  unshielded. Unshielded twisted pair is the most
  popular.

_
The twists in the cable reduce inductance while
the shielding protects the cable from outside
interference..
31
11.6 Network Organization

• Electronic Industries Alliance (EIA), along with
  the Telecommunications Industry Association (TIA)
  established a rating system called EIA/TIA-568B.
  
• The EIA/TIA category ratings specify the maximum
  frequency that the cable can support without
  excessive attenuation.
• The ISO rating system refers to these wire grades
  as classes.
• Most local area networks installed today are
  equipped with Category 5 or better cabling. Some
  are installing fiber optic cable.

32
11.6 Network Organization

• Optical fiber network media can carry signals
  faster and farther than either or twisted pair or
  coaxial cable.
• Fiber optic cable is theoretically able to
  support frequencies in the terahertz range, but
  transmission speeds are more commonly in the
  range of about two gigahertz, carried over runs
  of 10 to 100 Km (without repeaters).
• Optical cable consists of bundles of thin (1.5
  to 125 ?m) glass or plastic strands surrounded by
  a protective plastic sheath.

33
11.6 Network Organization

• Optical fiber supports three different
  transmission modes depending on the type of fiber
  used.
• Single-mode fiber provides the fastest data rates
  over the longest distances. It passes light at
  only one wavelength, typically, 850, 1300 or 1500
  nanometers.
• Multimode fiber can carry several different light
  wavelengths simultaneously through a larger fiber
  core.

34
11.6 Network Organization

• Multimode graded index fiber also supports
  multiple wavelengths concurrently, but it does so
  in a more controlled manner than regular
  multimode fiber
• Unlike regular multimode fiber, light waves are
  confined to the area of the optical fiber that is
  suitable to propagating its particular
  wavelength.
• Thus, different wavelengths concurrently
  transmitted through the fiber do not interfere
  with each other.

35
11.6 Network Organization

• Fiber optic media offer many advantages over
  copper, the most obvious being its enormous
  signal-carrying capacity.
• It is also immune to EMI and RFI, making it ideal
  for deployment in industrial facilities.
• Fiber optic is small and lightweight, one fiber
  being capable of replacing hundreds of pairs of
  copper wires.
• But optical cable is fragile and costly to
  purchase and install. Because of this, fiber is
  most often used as network backbone cable, which
  bears the traffic of hundreds or thousands of
  users.

36
11.6 Network Organization

• Transmission media are connected to clients,
  hosts and other network devices through network
  interfaces.
• Because these interfaces are often implemented on
  removable circuit boards, they are commonly
  called network interface cards, or simply NICs.
• A NIC usually embodies the lowest three layers of
  the OSI protocol stack.
• NICs attach directly to a systems main bus or
  dedicated I/O bus.

37
11.6 Network Organization

• Every network card has a unique 6-byte MAC (Media
  Access Control ) address burned into its
  circuits.
• The first three bytes are the manufacturer's
  identification number, which is designated by the
  IEEE. The last three bytes are a unique
  identifier assigned to the NIC by the
  manufacturer.
• Network protocol layers map this physical MAC
  address to at least one logical address.
• It is possible for one computer (logical address)
  to have two or more NICs, but each NIC will have
  a distinct MAC address.

38
11.6 Network Organization

• Signal attenuation is corrected by repeaters that
  amplify signals in physical cabling.
• Repeaters are part of the network medium (Layer
  1).
• In theory, they are dumb devices functioning
  entirely without human intervention. However,
  some repeaters now offer higher-level services to
  assist with network management and
  troubleshooting.

39
11.6 Network Organization

• Hubs are also Physical layer devices, but they
  can have many ports for input and output.
• They receive incoming packets from one or more
  locations and broadcast the packets to one or
  more devices on the network.
• Hubs allow computers to be joined to form network
  segments.

40
11.6 Network Organization

• A switch is a Layer 2 device that creates a
  point-to-point connection between one of its
  input ports and one of its output ports.
• Switches contain buffered input ports, an equal
  number of output ports, a switching fabric and
  digital hardware that interprets address
  information encoded on network frames as they
  arrive in the input buffers.
• Because all switching functions are carried out
  in hardware, switches are the preferred devices
  for interconnecting high-performance network
  components.

41
11.6 Network Organization

• Bridges are Layer 2 devices that join two similar
  types of networks so they look like one network.
• Bridges can connect different media having
  different media access control protocols, but the
  protocol from the MAC layer through all higher
  layers in the OSI stack must be identical in both
  segments.

42
11.6 Network Organization

• A router is a device connected to at least two
  networks that determines the destination to which
  a packet should be forwarded.
• Routers are designed specifically to connect two
  networks together, typically a LAN to a WAN.
• Routers are by definition Layer 3 devices, they
  can bridge different network media types and
  connect different network protocols running at
  Layer 3 and below.
• Routers are sometimes referred to as
  intermediate systems or gateways in Internet
  standards literature.

43
11.6 Network Organization

• Routers are complex devices because they contain
  buffers, switching logic, memory, and processing
  power to calculate the best way to send a packet
  to its destination.

44
11.6 Network Organization

• Dynamic routers automatically set up routes and
  respond to the changes in the network.
• They explore their networks through information
  exchanges with other routers on the network.
• The information packets exchanged by the routers
  reveal their addresses and costs of getting from
  one point to another.
• Using this information, each router assembles a
  table of values in memory.
• Typically, each destination node is listed along
  with the neighboring, or next-hop, router to
  which it is connected.

45
11.6 Network Organization

• When creating their tables, dynamic routers
  consider one of two metrics. They can use either
  the distance to travel between two nodes, or they
  can use the condition of the network in terms of
  measured latency.
• The algorithms using the first metric are
  distance vector routing algorithms. Link state
  routing algorithms use the second metric.
• Distance vector routing is easy to implement, but
  it suffers from high traffic and the
  count-to-infinity problem where an infinite loop
  finds its way into the routing tables.

46
11.6 Network Organization

• In link state routing, router discovers the speed
  of the lines between itself and its neighboring
  routers by periodically sending out Hello
  packets.
• After the Hello replies are received, the router
  assembles the timings into a table of link state
  values.
• This table is then broadcast to all other
  routers, except its adjacent neighbors.
• Eventually, all routers within the routing domain
  end up with identical routing tables.
• All routers then use this information to
  calculate the optimal path to every destination
  in its routing table.

47
11.7 High Capacity Digital Links

• Long distance telephone communication relies on
  digital lines.
• Because the human voice analog, it must be
  digitized before being sent over a digital
  carrier. The technique used for this conversion
  is called pulse-code modulation, or PCM.
• PCM relies on the fact that the highest frequency
  produced by a normal human voice is around
  4000Hz.
• Therefore, if the voices of a telephone
  conversation are sampled 8,000 times per second,
  the amplitude and frequency can be accurately
  rendered in digital form.

48
11.7 High Capacity Digital Links

• The figure below shows pulse amplitude modulation
  with evenly spaced (horizontal) quantization
  levels.
• Each quantization level can be encoded with a
  binary value.

This configuration conveys as much information by
each bit at the high end as the low end of the
4000Hz bandwidth.

49
11.7 High Capacity Digital Links

• However, a higher fidelity rendering of the human
  voice is produced when the quantization levels of
  PCM are bunched around the middle of the band, as
  shown below.

Thus, PCM carries information in a manner that
reflects how it is produced and interpreted.
50
11.7 High Capacity Digital Links

• Using127 quantization levels pulse-code
  modulation signal is distinguishable from a pure
  analog signal.
• So, the amplitude of the signal could be conveyed
  using only 7 bits for each sample.
• In the earliest PCM deployments, an eighth bit
  was added to the PCM sample for signaling and
  control purposes within the Bell System.
• Today, all 8 bits are used.
• A single stream of PCM signals produced by one
  voice connection requires a bandwidth of 64Kbps
  (8 bits ? 8,000 samples/sec.). Digital Signal 0
  (DS-0) is the signal rate of the 64Kbps PCM bit
  stream.

51
11.7 High Capacity Digital Links

• To form a transmission frame, a series of PCM
  signals from 24 different voice connections is
  placed on the line, with a control channel and
  framing bit forming a 125?s frame.
• This process is called time division multiplexing
  (TDM) because each connection gets roughly 1/24th
  of the 125?s frame.
• At 8,000 samples per second per connection, the
  combination of the voice channels, signaling
  channel and framing bit requires a total
  bandwidth of 1.544Mbps.

52
11.7 High Capacity Digital Links

• Europe and Japan use a larger frame size than the
  one that is used in North America.
• The European standard uses 32 channels, two of
  which are used for signaling and synchronization
  and 30 which are used for voice signals.
• The total frame size is 256 bits and requires a
  bandwidth of 2.048Mbps.
• The 1.544Mbps and 2.048Mbps line speeds are
  called T-1 and E-1, respectively, and they carry
  DS-1 signals.

53
11.7 High Capacity Digital Links

• DS-1 frames can be multiplexed onto high-speed
  trunk lines.
• The set of carrier speeds that results from these
  multiplexing levels is called the Plesiochronous
  Digital Hierarchy (PDH).
• As timing exchange signals propagate through the
  hierarchy, errors are introduced.
• The deeper the hierarchy, the more likely it is
  that the signals will drift or slip before
  reaching the bottom.

54
11.7 High Capacity Digital Links

• During the 1980s, BellCore and ANSI formulated
  standards for a synchronous optical network,
  SONET.
• The Europeans adapted SONET to the E-carrier
  system, calling it the synchronous digital
  hierarchy, or SDH.
• Just as the basic signal of the T-carrier system
  is DS-1 at 1.544Mbps, the basic SONET signal is
  STS-1 (Synchronous Transport System 1) at
  51.84Mbps.

55
11.7 High Capacity Digital Links

• When an STS signal is passed over an optical
  carrier network, the signal is called OCx, where
  x is the carrier speed.

The fundamental SDH signal is STM-1, which
conveys signals at a rate of 155.52Mbps. The
SONET hierarchy along with SDH is shown in the
table.

56
11.7 High Capacity Digital Links

• In 1982 the ITU-T completed a series of
  recommendations for the Integrated, Services
  Digital Network (ISDN), an all-digital network
  that would carry voice, video and data directly
  to the consumer.
• ISDN was designed in strict compliance with the
  ISO/OSI Reference Model.
• The ISDN recommendations focus on various network
  terminations and interfaces located at specific
  reference points in the ISDN model.

The organization of this system is shown on the
next slide.
57
11.7 High Capacity Digital Links
58
11.7 High Capacity Digital Links

• ISDN supports two signaling rate structures,
  Basic and Primary.
• A Basic Rate Interface consists of two 64Kbps
  B-Channels and one 16Kbps D-Channel.
• These channels completely occupy two channels of
  a T-1 frame plus one-quarter of a third one.
• ISDN Primary Rate Interfaces occupy the entire
  T-1 frame, providing 23 64Kbps B-Channels and the
  entire 64Kbps D-Channel.
• B-Channels can be multiplexed to provide higher
  data rates, such as 128Kbps residential Internet
  service.

59
11.7 High Capacity Digital Links

• Unfortunately, the ISDN committees were neither
  sufficiently farsighted nor fast enough in
  completing the recommendations.
• ISDN provides too much bandwidth for voice, and
  far too little for data.
• Except for a relatively small number of home
  Internet users, ISDN has become a technological
  orphan.
• The importance of ISDN is that it forms a bridge
  to a more advanced and versatile digital system,
  Asynchronous Transfer Mode (ATM).

60
11.7 High Capacity Digital Links

• ATM does away with the idea of time-division
  multiplexing.
• Instead, conversation and each data transmission
  consists of a sequence of discrete 53-byte cells
  that can be managed and routed individually to
  make optimal use of whatever bandwidth is
  available.
• Moreover, ATM is designed to be an efficient
  bearer service for digital voice, data, and video
  streams.
• In years since, ATM has been adapted to also be a
  bearer service for LAN and MAN services.

61
11.7 High Capacity Digital Links

• The CCITT called this next generation of digital
  services broadband ISDN, or B-ISDN, to emphasize
  its architectural connection with (narrowband)
  ISDN.
• ATM supports three transmission services
  full-duplex 155.52Mbps, full-duplex 622.08Mbps
  and an asymmetrical mode with an upstream data
  rate of 155.52Mbps and a downstream data rate of
  622.08Mbps.
• B-ISDN is downwardly compatible with ISDN. It
  uses virtually the same reference model, as shown
  on the next slide.

62
11.7 High Capacity Digital Links
63
11.8 A Look at the Internet

• We have described how the Internet went from its
  beginnings as a closed military research network
  to the open worldwide communications
  infrastructure of today.
• However, gaining access to the Internet is not
  quite as simple as gaining access to a dial tone.
  
• Most individuals and businesses connect to the
  Internet through privately operated Internet
  service providers (ISPs).

64
11.8 A Look at the Internet

• Each ISP maintains a switching center called a
  point-of-presence (POP).
• Some POPs are connected through high-speed lines
  (T-1 or higher) to regional POPs or other major
  intermediary POPs.
• Local ISPs are connected to regional ISPs, which
  are connected to national and international ISPs
  (often called National Backbone Providers, or
  NBPs).
• The NBPs are interconnected through network
  access points (NAPs).

The ISP-POP-NAP hierarchy is shown on the next
slide.
65
11.8 A Look at the Internet
66
11.8 A Look at the Internet

• Major Internet users, such as large corporations
  and government and academic institutions, are
  able to justify the cost of leasing direct
  high-capacity digital lines between their
  premises and their ISP.
• The cost of these leased lines is far beyond the
  reach of private individuals and small
  businesses.
• Consequently, Internet users with modest
  bandwidth requirements typically use standard
  telephone lines to serve their telecommunications
  needs.

67
11.8 A Look at the Internet

• Because standard telephone lines are built to
  carry analog (voice) signals, digital signals
  produced by a computer must first be converted,
  or modulated, from digital to analog form, before
  they are transmitted over the phone line.
• At the receiving end, they must be demodulated
  from analog to digital. A device called a
  modulator/ demodulator, or modem, converts the
  signal.
• Most home computers come equipped with built-in
  modems that connect directly to the system's I/O
  bus.

68
11.8 A Look at the Internet

• Modulating a digital signal onto an analog
  carrier means that some characteristic of the
  analog carrier signal is changed so that signal
  can convey digital information.
• Varying the amplitude, varying the frequency, or
  varying the phase of the signal can produce
  analog modulation of a digital signal.
• These forms of modulation are shown on the next
  slide.

69
11.8 A Look at the Internet
70
11.8 A Look at the Internet

• Using simple amplitude, frequency or 180?
  phase-change modulation, limits modem throughput
  to about 2400bps.
• Varying two characteristics at a time instead of
  just one increases the number of bits that can be
  transmitted.
• Quadrature amplitude modulation (QAM), changes
  both the phase and the amplitude of the carrier
  signal. QAM uses two carrier signals that are
  180? out of phase with each other.

71
11.8 A Look at the Internet

• Two waves can be modulated to create a set of
  Cartesian coordinates.
• The X,Y coordinates in this plane describe a
  signal constellation or signal lattice that
  encodes specified bit patterns.

A sine wave could be modulated for the
Y-coordinate and the cosine wave for the
X-coordinate.

72
11.8 A Look at the Internet

• Voice grade telephone lines are designed to carry
  a total bandwidth of 3000Hz.
• In 1924, information theorist Henry Nyquist
  showed that no signal can convey information at a
  rate faster than twice its frequency.
  Symbolically
• where baud is the signaling speed of the
  line.
• A 3000Hz signal can transmit two-level (binary)
  data at a rate no faster than 6,000 baud.

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• In 1948, Claude Shannon extended Nyquist's work
  to consider the presence of noise on the line,
  using the line's signal-to-noise ratio.
  Symbolically
• The public switched telephone network (PSTN)
  typically has a signal-to-noise ratio of 30dB.
• It follows that the maximum data rate of voice
  grade telephone lines is approximately 30,000bps,
  regardless of the number of signal levels used.

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11.8 A Look at the Internet

• The 30Kbps limit that Shannon's Law imposes on
  analog telephone modems is a formidable barrier
  to the promise of a boundless and open Internet.
  
• While long-distance telephone links have been
  fast and digital for decades, the local loop
  wires running from the telephone switching center
  to the consumer continues to use hundred-year-old
  analog technology.
• The "last mile" local loop, can in fact span many
  miles, making it extremely expensive to bring the
  analog telephone service of yesterday into the
  digital world of the present.

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11.8 A Look at the Internet

• The physical conductors in telephone wire are
  thick enough to support moderate-speed digital
  traffic for several miles without severe
  attenuation.
• Digital Subscriber Line (DSL) is a technology
  that can coexist with plain old telephone service
  (POTS) on the same wire pair that carries the
  digital traffic.
• At present, most DSL services are available only
  to those customers whose premises connect with
  the central telephone switching office (CO) using
  less than 18,000 feet (5,460 m) of copper cable.

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11.8 A Look at the Internet

• At the customer's premises, some DSLs require a
  splitter to separate voice from digital traffic.
  The digital signals terminate at a coder/decoder
  device often called a DSL modem.
• There are two differentand incompatible
  modulation methods used by DSL Carrierless
  Amplitude Phase (CAP) and Discrete MultiTone
  Service (DMT). CAP is the older and simpler of
  the two technologies, but DMT is the ANSI
  standard for DSL.

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11.8 A Look at the Internet

• CAP uses three frequency ranges, 0 to 4KHz for
  voice, 25KHz through 160KHz for "upstream"
  traffic (e.g., sending a command through a
  browser asking to see a particular Web page), and
  240KHz to 1.5MHz for "downstream" traffic
• This imbalanced access method is called
  Asymmetric Digital Subscriber Line (ADSL).
• The fixed channel sizes of CAP lock in an
  upstream bandwidth of 135KHz.
• This may not be ideal for someone who does a
  great deal of uploading, or connects to a remote
  LAN.

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11.8 A Look at the Internet

• Where a symmetric connection is required,
  Discrete MultiTone DSL may offer better
  performance.
• DMT splits a 1MHz frequency bandwidth into 256
  4KHz channels, called tones.
• These channels can be configured in any way that
  suits both the customer and the provider.
• DMT can adapt to fluctuations in line quality.
• When DMT equipment detects excessive crosstalk or
  excessive attenuation on one of its channels, it
  stops using that channel until the situation is
  remedied.

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11.8 A Look at the Internet

• The analog local loop one of the problems facing
  the Internet today.
• A more serious problem concerns backbone router
  congestion.
• More than 50,000 routers serve various backbone
  networks in the United States alone.
• Considerable time and bandwidth is consumed as
  the routers exchange routing information.
• Obsolete routes can persist long enough to impede
  traffic, causing even more congestion as the
  system tries to resolve the error.

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11.8 A Look at the Internet

• Greater problems develop when a router
  malfunctions, broadcasting erroneous routes (or
  good routes that it subsequently cancels) to the
  entire backbone system.
• This is known as the router instability problem
  and it is an area of continuing research.
• When IPv6 is adopted universally some of these
  problems will go away because the routing tables
  ought to get smaller.

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11.8 A Look at the Internet

• Even with improved addressing, there are limits
  to the speed with which tens of thousands of
  routing tables can be synchronized.
• This problem is undergoing intense research, the
  outcome of which may give rise to a new
  generation of routing protocols.
• One thing is certain, simply giving the Internet
  more bandwidth offers little promise for making
  it any faster in the long-term.
• It has to get smarter.

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Chapter 11 Conclusion

• The ISO/OSI RM describes a theoretical network
  architecture. This architecture has to some
  extent been incorporated into digital
  telecommunication systems, including ISDN and
  ATM.
• TCP/IP using IPv4 is the protocol supported by
  the Internet. IPv6 has been defined and
  implemented by numerous vendors, but its adoption
  is incomplete.

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Chapter 11 Conclusion

• Network organization consists of physical (or
  wireless) media, NICs, modems, CSU/DSUs,
  repeaters, hubs, switches, routers, and
  computers. Each has its place in the OSI RM.
• Many people connect to the Internet through dial
  up lines using modems. Faster speeds are provided
  by DSL.
• The Internet is a hierarchy of ISPs, POPs, NAPs,
  and various backbone systems.
• The router instability problem is one of the
  largest challenges for the Internet.

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End of Chapter 11